Colour television display apparatus provided with a cathode-ray tube

ABSTRACT

Colour television display apparatus in which for correction of isotropic astigmatism a correction current flows through the coil halves of at least one deflection coil unit, which current is substantially a square function of at least one deflection current. As a result a correction quadripolar field is produced. The correction current generator includes passive elements, namely impedances, diodes and transistors operating as switches. The deflection coil halves are toroidally wound on the core.

llnlted States Patent 1191 1111 3,824,426 Rossaert July 16, 1974 COLOUR TELEVISION DISPLAY 2,906,919 9/1959 Thor et a1. 315/27 SR APPARATUS PROVIDED WITH A 3,341,735 9/1967 Briggs 315/27 GD Hansen et al. 3 X

' 3,714,500 1/1973 Kaashoek 315/27 01) x Inventor: Edgard Emile Charles Rossaert,

Brussels, Belgium Assignee: I U.S. Philips Corporation, New

York, NY.

Filed: Oct. 11, 1972 Appl. No.: 296,657

Foreign Application Priority Data Oct. 23, 1971 Netherlands 7114607 US. Cl 315/27 GD, 315/13 C, 315/13 CG, 3'15/27 TD Int. Cl. H01j 29/70 Field of Search 315/13 C, 13 CG, 27 TD,

' 315/27 X, 27 GD References Cited UNITED STATES PATENTS Sanford 315/27 GD Primary ExaminerBenjamin R. Padgett Assistant Examiner-P. A. Nelson Attorney, Agent, or FirmFrank R. Trifari; Henry 1.

Steckler [5 7] ABSTRACT Colour television display apparatus :in which for correction of isotropic astigmatism a correction current flows through the coil halves of at least one deflection coil unit, which current is substantially a square functionof at least one deflection current. As a result a correction quadripolar field is produced. The correction current generator includes passive elements, namely impedances, diodes and transistors operating as switches. The deflection coil halves are toroidally wound on the core.

I 10 Claims, 8 Drawing Figures PATENTEU JUL 1 61974 .SHEET 2 OF 4 COLOUR TELEVISION DISPLAY APPARATUS PROVIDED WITH A CATHODE-RAY TUBE The invention relates to colour television display apparatus provided with a cathode-ray tube including a display screen and a system of deflection coils comprising a magnetic core on which a first and a second deflection coil unit are provided, each unit comprising two preferably symmetrical coil halves, said system of deflection coils being slid on the neck of the cathoderay tube for deflecting in two substantially orthogonal directions at least one electron beam generated in the cathode-ray tube because a deflection current originating from a deflection current generator flows through each coil half, while for correction of the isotropic astigmatism a quadripolar field is generated approximately at the area of the deflection plane of the electron beam, the intensity of said field being substantially a square function of the instantaneous intensity of at least one of the deflection currents, the apparatus furthermore comprising a correction current generator for providing a correction current for generating the said quadripolar field.

Such apparatus is described in U.S. Pat. application No. 52,640, filed July 6, 1970. In this apparatus the deflecttion coil system is provided with at least four toroid auxiliary windings through which the correction current flows which is generated by a generator. This generator must apply sawtooth and parabolic signals to the said windings and includes active amplifier elements, namely transistors to which supply power is to be applied.

The relevant invention has for its object to provide improved and simplified apparatus in which substantially no supply power is required. To this end the apparatus according to the invention is characterized in that the correction current generator consists of passive elements, the correction. current flowing through the coil halves of at least one deflection coil unit and the deflection coil halves being toroidally wound on the core.

Since the correction current generator consists of passive elements, the deflection current must flow through these elements so that the correction current is superimposed on the deflection current.

It may be noted that it is known per se from U.S. Pat.

Specification No. 3,440,483 to cause a correction current for generating a quadripolar field to flow through deflection coils. The point there is, however, a correction of anisotropic astigmatic deflection errors so that the correction current is dependent on the product of the instantaneous intensity of the two deflection currents which in addition necessitates a modulator.

It is also known per se from German Pat. application 1,514,896 to eliminate astigmatic errors by means of passive elements. These elements are, however, used to render the field generated by one deflection coil unit dependent on the field generated by the other deflection coil unit. Otherwise the point is the deflection fields themselves and not correction fields while the said dependency is established by means of separate auxiliary windings on the core.

Some possible embodiments of display apparatus according to the invention will be described in detail by way of example with reference to the accompanying Figures in which: I

FIG. 1 shows display apparatus provided with a three gun cathode-ray tube and switching means for supplying the correction current,

FIG. 2 shows an embodiment of the said switching means,

FIG. 3 shows characteristic curves of part of the embodiment according to FIG. 2, and

FIGS. 4, 5, 6, 7 and 8 show further embodiments of the said switching means.

In FIG. 1, 1 denotes an aerial by which the colour television signal can be received. This colour television signal is applied to an RF and IF amplifier 2 which am plifies the signal, detects it and subsequently applies it to a video amplifier 3. This video amplifier 3 applies the actual video signal consisting of a luminance signal and colour difference signals to a first output 4. These signals are processed in a matrix circuit 5 so that the three colour signals R, G and B become available at the output of this matrix circuit and are applied to the three cathodes K K and K of the cathode-ray tube 6 operating as a colour television display tube. This tube 6 may be of the shadow-mask type and the colour image is displayed on its screen. The synchronizing signal, which is applied at one end to the line deflection generator 8 and at the other end to the field deflection generator 9, is derived from a second output 7 of video amplifier 3. Two output terminals 10 and 11 of generator 8 at one end are connected to the deflection coil system 12 and an output terminal 13 at the other end is connected to the final anode of display tube 6 for the supply of the final anode voltage of approximately 25 kilovolts. The output terminals 14 and 15 of field deflection generator 9 are likewise connected to deflection coil system 12 for the supply of the field deflection current. As a rule the line deflection current derived from output terminals 10 and 11, combined with a deflection unit of deflection coil system 12 ensures the deflection of the electron beams generated by the three cathodes K K and K in the horizontal direction. Simultaneously the field deflection current derived from output terminals 14 and 15, in co-operation with a further deflection unit of deflection coil system 12, ensures the deflection of the three electron beams in the vertical direction.

According to one aspect of the invention output terminals 10 and 11 and 14 and 15 are not directly connected to deflection coil system 12, but through arrangements 16 and 16, respectively. FIG. 2 shows an embodiment of arrangement 16. Windings l7 and 18 are the coil halves of the first-mentioned deflection unit, i.e. for the horizontal deflection. Arrangement 16 includes two impedances Z and Z each of which is arranged in series with one of the said windings. Line deflection generator Sapplies the line deflection current i to the circuit according to FIG. 2. When E is referred to as the voltage between outputs 10 and 1 1, and when Z, is the impedance of both winding 17 and winding 18 and Z is the mutual impedance between windings l7 and 18, then:

In fact, impedance Z is to be considered as negative because the fields generated by the currents flowing in windings 17 and 18 support each other in the space within the core and therefore have opposite directions in the core. This applies to toroidally wound windings. When the condition is imposed that:

Z; Z AZ Z Z AZ,

the first formula becomes EH o' H) M a+ H+ M) n- A current flows through winding 17:

i I ZO+ZH+ZM+AZ/ Z0+ZH-) ZML( 11 and a current flows through winding 18:

i Z +Z +Z AZ/[(Z,,+Z +Z (AZ) E If it is assumed for the sake of simplicity that (AZ) is negligibly small relative to (Z +Z 2 we can write:

in which Al in.

When impedance AZ varies proportionally to current i it is found that current A1), is proportional to the square of current i jUnder these circumstances windings 17 and 18 not only generate the magnetic field for the horizontal deflection but also a quadripolar field. According to a further aspect of the invention the deflection coils must be toroidally wound on the core of deflection coil system 12 with the deflection coil halves being located symmetrically relative to the vertical deflection direction. The result is that the poles of the quadripolar field are located substantially in the direction of the diagonals of the angle constituted by the horizontal and vertical deflection directions. As is further described in Netherlands Pat. application 6910495 the described apparatus is therefore suitable to correct isotropic astigmatic deflection errors which have been caused by deflection coil system 12 upon deflection in the horizontal direction.

It is found from the above-mentioned formula that current Ai is many times smaller than current i so that the generated correction quadripolar field has a low intensity. An increase thereof is, however, obtained if a second quadripolar field is generated by a current flowing through the field deflection coils which is induced by current Ai, while the total induced voltage is zero. All this has been described in greater detail in U.S. Pat. application No. 294,912, Oct. 4, 1972 which also states that toroidally wound deflection coils are advantageous for this purpose.

lmpedances Z and Z for which there applies that Z Z AZ and Z Z AZ while AZ is proportional to the current flowing through the impedances may be formed as coils having a saturable core. FIG. 3a shows the BH characteristic curve of the material of which this core is made in which B the magnetic induction and H is the magnetic field strength. FIG. 3b shows the pH characteristic curve thereof in which p. dB/dI-I is the permeability. At a field strength H and H,, the material is saturated while for a deviation iAH about field strengths H and H,, the BH characteristic curve has a substantially square variation. It follows that the uI-I characteristic curve for the same deviation about field strengths H, and H has a substantially linear variation. When the field strength varies from the value H to the value H AH, the permeability varies in a linear manner from a value n to a value u Au, while a variation of the field strength from the value H,, to the value H,, AH results in a linear variation of the permeability from the value ,u.., to the value p. An. Since field strength H is proportional to current i and since the impedance of the coil is proportional to its inductance and therefore to the permeability of the core, such a coil is indeed suitable for the envisaged purpose. Field strengths H and -H may be adjusted by means of two permanent rotatable magnets in the absence of current i Alternatively the arrangement diagrammatically shown in FIG. 4 may be used with two cores 19 and 20 whose primary windings constitute the impedances Z and Z while a direct current I flows through secondary windings 21 and 22, which current is adjustable by means of a battery 23 and a variable resistor 24. FIG. 4 shows that current I provides a positive premagnetisation for one of the cores l9 and 20 and a negative premagnetisation for the other core.

The latter arrangement is not usable in combination with the deflection coils for the vertical deflection because these coils are predominantly resistive for the field frequency. FIG. 5 shows another embodiment of circuits l6 and 16' of FIG. 1 in which in this embodiment the vertical deflection is corrected. Deflection generator 9 supplies the field deflection current i Circuit 16' includes two diodes D, and D and an impedance Z which are arranged as shown in FIG. 5. When the diodes are considered as ideal switches it can be assumed that one diode conducts during one half of the sweep of the sawtooth deflection current and then constitutes a short circuit while the other diode is blocked. During the other half of the sweep the reverse situation occurs. Therefore the deflection current always flows through impedance Z.

When Z is the impedance of both winding 25 and winding 26, which constitute the coil halves of the deflection unit for the vertical deflection and when Z M is the mutual impedance therebetween, a current flows through winding 25:

i iv

and a current flows through winding 26: i (Z 'i-Z+Z' /2Zri-Z+2Z' i when diode D conducts.

This may be expressed as follows:

i 2 i in which i V2 Z/2Z +Z+2Z i kiy, that is to say, Ai is proportional to i Under these circumstances the deflection coil halves not only generate the deflection field but also a quadripolar field whose pole axes are located approximately in the same manner as was the case for the arrangement according to FIG. 2, provided that the deflection coils are toroidally wound and are placed symmetrically relative to the horizontal and/or vertical deflection directions. The field strength thereof is proportional to the deflection current and can therefore be represented as a symmetrical triangle as a function of time, which may be considered as an approximation of a parabola. The

describedarrangement is therefore suitable to correct isotropic astigmatic deflection errors which are caused by the deflection system 12 in the direction corresponding to the relevant deflection coil halves.

If the arrangement according to FIG. relates to the horizontal deflection, coil halves 17 and 18 have an impedance Z R jwL in which R and L are the resistance and the inductive part, respectively, of impedance Z Since the proportionality constant k is a real number, impedance Z must be equal to Z R jwL with L/R z L /R while the choice of the absolute value of k depends on the desired slope of the mentioned triangular function. For the vertical deflection coil halves 25 and 26 are predominantly resistive so that impedance Z must be a predominantly resistive impedance. 7

FIG. 6 shows a refinement of the arrangement according to FIG. 5 in which diodes D and D are replaced by two transistors T and T of opposite types. These transistors do not operate as amplifiers but as switches and are either saturated or cut off while the conducting and cut-off periods are adjustable by means of two supply voltages +V and V,, and two base resistors so that their function is the same as that of diodes D and D of FIG. 5 and, likewise as these diodes, may be considered as passive elements. The adjustment of transistors T and T may be such that the two transistors conduct about the centre of the sweep so that the field strength of the correction quadripolar field as a function of time is trapezoid-shaped, which is a better approximation of the desired parabola shape. Transistors have the additional advantage that the voltage drop thereacross in case of saturation is lower than in diodes.

FIG-7 shows a modification of the apparatus according to the invention in which the deflection coil halves are arranged in series. Winding 27 is a secondary winding of an output transformer associated with deflection generators 8 and 9. Two preferably identical series arrangements of diodes D' and D' and of an impedance Z are arranged between the junction N of deflection coil halves 17 and 18 and 25 and 26, respectively, and two taps P and P which are located symmetrically relative to the central tap M of winding 27. Diodes D' and D may be replaced bytransistors in a corresponding manner as described hereinbefore. Central tap M conveys the same potential as point N so that one diode conducts while the other is blocked. Also in this case it can be shown that the current flowing through one coil half is the sum of the deflection current and a current proportional to the deflection current and that the current flowing through the other coil half is the difference between the deflection current and the same current. This is the same situation as in the arrangement according to FIG. 5 and it has the same result in so far as the construction of impedance Z is concerned.

FIG. 8 shows a complete diagram of the apparatus according to the invention in which a circuit according to that of FIG. 2 is used for the horizontal deflection and a circuit according to that of FIG. 5 is used for the vertical deflection. As a result a correction of isotropic astigmatic errors in the two deflection directions is obtained. An important improvement is obtained in that a second quadripolar field is generated by a current induced in the second deflection unit by a correction current flowing in the first deflection unit, while the total induced voltage is Zero as described in the above- What is claimed is:

1. A circuit for a system of deflection coils comprising a magnetic core, first and second deflection units, each unit comprising two coil halves toroidally wound on said core, said circuit comprising a pair of deflection current generator means for generating two substantially linear deflection currents, means coupling said generator means to said units respectively; and means for correcting isotropic astigmatism comprising means for generating a quadripolar field having an intensity that is substantially a square function of one of the deflection currents, said quadripolar generating means comprising a passive element correction current generator means, and means for coupling said passive correction means to at least one deflection coil unit for providing a correction current through the halves thereof.

2. A circuit as claimed in claim 1 wherein one of said units comprises a horizontal deflection unit, said correction generator means comprising a pair of equal inductance saturable coils having magnetic fields of opposite directions in the absence of the horizontal deflection current, said saturable coils being series coupled to said horizontal coil halves respectively whereby series circuits are formed, said series circuits being parallel coupled together.

3. A circuit as claimed in claim lwherein said correction generator means comprises a pair of unidirectional conducting elements series coupled to said halves of oneof said units respectively, whereby two series circuits and a pair of junctions between said elements and halves are formed respectively, said series circuits being parallel coupled together, said elements having opposite conductivity directions, and an impedance means coupled between said junctions.

4. A circuit as claimed in claim 1 wherein at least two of said halves are series coupled together, whereby a series arrangement having a junction is formed, and said correction generator means comprises a winding coupled to said series arrangement and having a pair of taps symmetrical relative to the center of said winding, a pair of series circuits each comprising of a unidirectional conducting element and an impedance means series coupled thereto, said series circuits being coupled between said junction and said taps respectively.

5. A circuit as claimed in claim 4 wherein each of said elements comprises an adjustable conducting period transistor.

6. A circuit as claimed in claim 3 wherein each of said elements comprises an adjustable conducting period transistor.

7. A circuit as claimed in claim 4 wherein each of said coil halves have resistance and reactance and said impedance means comprises an inductor having resistance and reactance in a ratio substantially equal to the corresponding ratio in said halves.

7 8 8. A circuit as claimed in claim 3 wherein each of 9. A circuit as claimed in claim 1 further comprising said coil halves having resistance and reactance and said deflection units. said impedance means comprises an inductor having 10. A circuit as claimed in claim 9 wherein said coil resistance and reactance in a ratio substantially equal halves are symmetrical. to the corresponding ratio in said halves. 

1. A circuit for a system of deflection coils comprising a magnetic core, first and second deflection units, each unit comprising two coil halves toroidally wound on said core, said circuit comprising a pair of deflection current generator means for generating two substantially linear deflection currents, means coupling said generator means to said units respectively; and means for correcting isotropic astigmatism comprising means for generating a quadripolar field having an intensity that is substantially a square function of one of the deflection currents, said quadripolar generating means comprising a passive element correction current generator means, and means for coupling said passive correction means to at least one deflection coil unit for providing a correction current through the halves thereof.
 2. A circuit as claimed in claim 1 wherein one of said units comprises a horizontal deflection unit, said correction generator means comprising a pair of equal inductance saturable coils having magnetic fields of opposite directions in the absence of the horizontal deflection current, said saturable coils being series coupled to said horizontal coil halves respectively whereby series circuits are formed, said series circuits being parallel coupled together.
 3. A circuit as claimed in claim 1 wherein said correction generator means comprises a pair of unidirectional conducting elements series coupled to said halves of one of said units respectively, whereby two series circuits and a pair of junctions between said elements and halves are formed respectively, said series circuits being parallel coupled together, said elements having opposite conductivity directions, and an impedance means coupled between said junctions.
 4. A circuit as claimed in claim 1 wherein at least two of said halves are series coupled together, whereby a series arrangement having a junction is formed, and said correction generator means comprises a winding coupled to said series arrangement and having a pair of taps symmetrical relative to the center of said winding, a pair of series circuits each comprising of a unidirectional conducting element and an impedance means series coupled thereto, said series circuits being coupled between said junction and said taps respectively.
 5. A circuit as claimed in claim 4 wherein each of said elements comprises an adjustable conducting period transistor.
 6. A circuit as claimed in claim 3 wherein each of said elements comprises an adjustable conducting period transistor.
 7. A circuit as claimed in claim 4 wherein each of said coil halves have resistance and reactance and said impedance means comprises an inductor having resistance and reactance in a ratio substantially equal to the corresponding ratio in said halves.
 8. A circuit as claimed in claim 3 wherein each of said coil halves having resistance and reactance and said impedance means comprises an inductor having resistance and reactance in a ratio substantially equal to the corresponding ratio in said halves.
 9. A circuit as claimed in claim 1 further comprising said deflection units.
 10. A circuit as claimed in claim 9 wherein said coil halves are symmetrical. 